On the effect of steam in fluidized bed calcium looping processes

Carbon dioxide capture and storage has recently received increasing attention as a mean to moderate environmental problems related to CO2 emissions into the atmosphere. In this scenario, the Calcium Looping (CaL) process carried out in dual interconnected fluidized bed systems is among the most promising techniques to treat CO2-containing combustion flue gases. CaL is based on alternated temperature-swing CO2 uptake, with absorption taking place in a carbonator operated at around 650–700°C, followed by release of concentrated CO2 in a calciner operated at around 900–950°C, according to the reversible reaction CaO(s)+CO2(g)=CaCO3(s). A somehow less investigated aspect of CaL is represented by the possible effect of steam. Steam can be present in a CaL process in both the carbonator, where the flue gas coming from a combustion unit and contacting the sorbent may contain significant amounts of it, and the calciner, which is typically operated as an oxyfuel combustor to drive the endothermal calcination reaction, hence with appreciable partial pressure of steam. Recent studies highlighted that, during high-temperature calcination, steam may induce relevant changes in the micro-texture of sorbent calcines. Moreover, tests carried out exposing a sorbent to steam during the carbonation stage indicated that steam can “catalyze” the heterogeneous CaO/CO2 reaction by enhancing the CO2 diffusion across the CaCO3-rich shell of the sorbent particles. This study aims at improving the current level of understanding of the influence of steam on the development of textural and microstructural properties of a limestone-based sorbent upon iterated carbonation/calcination cycles. To this end, an experimental campaign has been performed in a lab-scale fluidized bed reactor. The campaign consisted in a reference test without steam, whose results are compared with tests in which sorbents were exposed to steam in either the calcination or the carbonation stages, or in both.